One or more deformations present within a sheet of a medium to be printed can cause serious reliability problems in a printing system, such as an inkjet printing system, especially if there is only a small gap provided between a sheet transport mechanism and an image forming device or printing head of the printing system. If the sheet to be printed touches the image forming device or the printing head as a result of such a deformation, this can lead to print quality degradation and/or to a sheet jam in the machine.
To address these issues, systems have been developed which employ a proofing device capable of identifying sheet deformations and rejecting sheets that contain such deformations. However, there are many sources of defects or errors that may lead to sheets being rejected which degrade the productivity of a printing system. For example, the sheets to be printed supplied to a printing machine may already contain various defects. Also, defects and wear within the machine can cause the sheets to become damaged. Changes in the environmental conditions can lead to deformation of the sheets as they are being processed, and inappropriate settings in a printing system, such as too much ink or a drying temperature that is too high, can also generate problems. Furthermore, such influences or defects can act in combination, making it very difficult to identify a root cause of a problem.
Defect detection units for a printing system known from the prior comprise a detector for sensing a single type of defect, for example a wrinkle detector or a dog ear detector. Based on the presence of a defect, a signal is emitted, indicating that the sheet is unsuited for printing. Sheets comprising a variety of defect types are often incorrectly designated as either suited or unsuited via the defection detection units in the prior art.
US2004100016 A describes a media qualification device wherein sheets may be sorted among a plurality of usable media paths according to media grade. The media qualification device may further comprise an input/output device configured to select media grade parameters.